Method for controlling the compression of an incoming feed air stream to a cryogenic air separation plant

ABSTRACT

A method for controlling the compression of an incoming feed air stream to a cryogenic air separation plant using at least two variable speed compressor drive assemblies controlled in tandem is provided. The first variable speed driver assembly drives at least one compression stage in the lower pressure compressor unit driven while the second variable speed driver assembly drives higher pressure compression stage disposed either in the common air compression train or the split functional compression train of the air separation plant. The first and second variable speed driver assemblies are preferably high speed, variable speed electric motor assemblies each having a motor body, a motor housing, and a motor shaft with one or more impellers directly and rigidly coupled to the motor shaft via a sacrificial rigid shaft coupling.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part (CIP) application andclaims the benefit of and priority to U.S. patent application Ser. No.13/644,066 filed on Oct. 3, 2012.

TECHNICAL FIELD

The present invention relates to methods of controlling the compressionof an incoming feed air stream to a cryogenic air separation plant, andmore specifically, to a method for controlling compression of anincoming feed air stream using at least two direct drive compressionassemblies controlled in tandem.

BACKGROUND

Cryogenic air separation is a very energy intensive process because ofthe need to generate high pressure, very low temperature air streams andthe large amount of refrigeration needed to drive the process. In atypical cryogenic air separation plant, an incoming feed air stream ispassed through a main air compressor (MAC) arrangement to attain adesired intermediate discharge pressure and flow. Prior to suchcompression, dust and other contaminants are typically removed from theincoming feed air stream via an air filter typically disposed in an airsuction filter house. The filtered air stream is compressed in amulti-stage MAC compression arrangement typically to a minimum pressureof about 6 bar and often at higher pressures. The compressed, incomingfeed air stream is then purified in a pre-purification unit to removehigh boiling contaminants from the incoming feed air stream. Such apre-purification unit typically has beds of adsorbents to adsorb suchcontaminants as water vapor, carbon dioxide, and hydrocarbons. In manyair separation plants the compressed, purified feed air stream orportions thereof are further compressed in a series of booster aircompressor (BAC) arrangements to even higher discharge pressures. Inconventional air separation plants, the MAC compression arrangements arelocated upstream of pre-purification unit whereas the BAC arrangementsare located downstream of pre-purification unit.

The compressed or further compressed, purified feed air streams are thencooled and separated into oxygen-rich, nitrogen-rich, and argon-richfractions in a plurality of distillation columns that may include ahigher pressure column, a lower pressure column, and optionally, argoncolumn (not shown). As indicated above, prior to such distillation thecompressed, pre-purified feed air stream is often split into a pluralityof compressed, pre-purified feed air streams, some or all of which arethen passed to a multi-stage BAC compression arrangement to attain thedesired pressures required to boil the oxygen produced by thedistillation column system. The plurality of compressed, pre-purifiedfeed air streams including any further compressed, pre-purified feed airstreams are then cooled within the primary or main heat exchanger totemperatures suitable for rectification in the distillation columnsystem. The sources of the cooling the plurality of feed air streams inthe primary heat exchanger typically include one or more waste streamsgenerated by the distillation column system as well as any supplementalrefrigeration generated by the cold turbine and warm turbinearrangements, described below.

The plurality of cooled, compressed air streams are then directed totwo-column or three column cryogenic air distillation column systemwhich includes a higher pressure column thermally linked or coupled to alower pressure column, and an optional argon column. Prior to enteringthe higher pressure column and lower pressure columns, any liquid airstreams may be expanded in a Joule-Thompson valve to produce stillfurther refrigeration required for producing the cryogenic products,including liquid oxygen, liquid nitrogen and/or liquid argon.

In air separation units designed to produce a large amount of liquidproducts, such as liquid oxygen, liquid nitrogen and liquid argon, alarge amount of supplemental refrigeration must be provided, typicallythrough the use of Joule-Thompson valves, described above, cold turbinearrangements and/or warm recycle turbine arrangements. Cold turbinearrangements are often referred to as either a lower column turbine(LCT) arrangement or an upper column turbine (UCT) arrangement which areused to provide supplemental refrigeration to a two-column or threecolumn cryogenic air distillation column system. On the other hand, awarm recycle turbine (WRT) arrangement expands a refrigerant stream in awarm turbo-expander with the resulting exhaust stream, cooled viaexpansion of the refrigerant stream, imparting supplementalrefrigeration to the cryogenic air distillation column system viaindirect heat exchange with the pre-purified, compressed feed air in theprimary heat exchanger or in an auxiliary heat exchanger.

In the LCT arrangement, a portion of the pre-purified, compressed feedair is further compressed in a BAC compression arrangement, partiallycooled in the primary heat exchanger, and then all or a portion of thisfurther compressed, partially cooled stream is diverted to aturbo-expander, which may be operatively coupled to and drive acompressor. The expanded gas stream or exhaust stream is then directedto the higher pressure column of a two-column or three column cryogenicair distillation column system. The supplemental refrigeration createdby the expansion of the diverted stream is thus imparted directly to thehigher pressure column thereby alleviating some of the cooling duty ofthe primary heat exchanger.

Similarly, in the UCT arrangement, a portion of the purified andcompressed feed air is partially cooled in the primary heat exchanger,and then all or a portion of this partially cooled stream is diverted toa warm turbo-expander, which may also be operatively coupled to anddrive a compressor. The expanded gas stream or exhaust stream from thewarm turbo-expander is then directed to the lower pressure column in thetwo-column or three column cryogenic air distillation column system. Thecooling or supplemental refrigeration created by the expansion of theexhaust stream is thus imparted directly to the lower pressure columnthereby alleviating some of the cooling duty of the primary heatexchanger.

The MAC compression arrangement and BAC compression arrangement requiresignificant amount of power to achieve the required compression.Typically, the MAC compression arrangement consumes roughly 60% to 70%of the total power consumed by the air separation plant. While a portionof the air separation plant power requirement may be recovered via theabove-described cold turbine arrangement and/or warm turbine arrangementwhich provide the supplemental refrigeration to the two-column or threecolumn cryogenic air distillation column system, the vast majority ofthe power required by the air separation plant is externally suppliedpower to drive the multi-stage MAC compression arrangement and themulti-stage BAC compression arrangement.

Most conventional MAC compression arrangements and BAC compressionarrangements as well as nitrogen recycle compressors and related productcompressors are configured as an integrally geared compressor (IGC)arrangements that include one or more compression stages coupled to asingle speed driver assembly, and a gearbox adapted for driving the oneor more of the compression stages via a bull gear and associated pinionshafts such that all pinion shafts operate at constant speed ratios. Theone or more compression stages typically use a centrifugal compressor inwhich the feed air entering an inlet is distributed to a vanedcompressor wheel known as an impeller that rotates to accelerate thefeed air and thereby impart the energy of rotation to the feed air. Thisincrease in energy is accompanied by an increase in velocity and apressure rise. The pressure is recovered in a static vaned or vanelessdiffuser that surrounds the impeller and functions to decrease thevelocity of the feed air and thereby increase the pressure of the feedair. The impellers may be arranged either on multiple shafts or on asingle shaft coupled to the single speed driver. Where multiple shaftsare used, a gearbox and associated lube oil system are typicallyrequired.

The conventional MAC compression arrangements further require aplurality of intercoolers provided between the multiple stages of thecompressor to remove the heat of compression from the compressed airstream between each compression stage. The reason for this is as the airis compressed, its temperature rises and the elevated air temperaturerequires an increase in power to compress the gas. Thus, when the air iscompressed in stages and cooled between stages, the compression powerrequirement is reduced due to closer to isothermal compression comparedto compression without interstage cooling. An aftercooler, such as adirect contact aftercooler, or air chiller are also typically positionedbetween the MAC compression arrangement and BAC compression arrangement.

It has been suggested to replace portions of the conventional IGCarrangements with a direct drive compressor assembly arrangement. Thedirect coupling of the compressor and the driver assembly overcomes theinefficiencies inherent in a gear box arrangement in which thermallosses occur within the gearing between the driver assembly and thecompression stages. Such a direct coupling is known as a direct drivecompressor assembly where both driver assembly shaft and the impellerrotate at the same speed. Typically such direct drive compressorassemblies are capable of variable speed operation. A direct drivecompressor assembly can thereby be operated to deliver a range of flowrates through the multiple compression stages and a range of pressureratios across the compressor units by varying the driver speed.

In addition, most conventional MAC compression arrangements are designedto be optimized at a design point corresponding to a point at or nearpeak flow capacity. However, in many air separation plants, it has beenfound that compressors typically operate at their respective designconditions less than 10% of the time and, in some plants, less than 5%of the time. The peak flow capacity of the MAC compression arrangementand BAC compression arrangement will be limited by centrifugal impellersize that can be manufactured by compressor manufacturers and theallowable impeller tip speed. In conventional systems, all MACcompression stages are often driven by the same power train or driver.Therefore, once a design speed is selected for this MAC driver, there islittle room to change the speed, since any speed change will impact allof the MAC compression stages as well as any of the BAC compressionstages that may be also coupled to the same power train. Using thistraditional design point, conventional MAC compression arrangements canoften achieve a turndown (i.e. decreasing the flow rate of the air thatis compressed) of only about 30% turndown using inlet guide vanesassociated with one or more of the compression stages.

For any given air separation plant, while the air inlet pressure isgenerally constant, the ambient air inlet temperature can varysignificantly from winter to summer, or even from day to night, leadingto considerable variation in volumetric flow. Once the design speed isselected, there is little room to change this speed to accommodateseasonal temperature and/or production changes. Thus, the most effectivecompressor performance control variable, i.e., driver speed, is not adegree of freedom to use for operational control of most conventionalMAC and BAC compression arrangements.

For example, to handle the required flow and the head for the summerhigh temperature condition, the MAC compression arrangement will need tobe sized for the summer high temperature condition and inlet guide vaneswill be partially closed to handle normal operating conditions. Thiscould reduce the compressor efficiency for other operating conditionsand also reduce the plant turndown range (i.e. the range from the designflow to the minimum allowable flow without compressor surge). Duringturndown conditions, the volumetric flow is reduced and therefore, theinlet guide vanes have to be closed further and, in some cases,compressed air may have to be vented to the atmosphere to prevent thecompressors from surging. Closing of the inlet guide vanes and/orventing a portion of the compressed air both translate to waste of powerand a decrease in overall plant efficiency.

Also, to optimize the air separation cycles, the compression trains ofmost air separation plants, including plants using direct drivecompression assemblies as part of the air compression trains, aredesigned to provide a desired volumetric flow to the pre-purificationunit in the case of the MAC compression arrangement or pressuresrequired by the distillation column system in the case of a BACcompression arrangement. Maintaining a generally constant dischargepressure in such air separation plants may also translate to waste ofpower and a decrease in overall plant efficiency across all operatingconditions. There is also a need to allow for continual or periodicadjustments to the incoming feed air flow capacity and/or dischargepressures of the air compression trains without sacrificing overall airseparation plant efficiency.

Accordingly, there is a continuing need to reduce the operating costs,namely power costs, associated with air compression arrangements in anair separation plant by employing effective direct drive compressionassemblies as part of the air compression trains. Prior art systems thatemploy direct drive compression assemblies as part of the aircompression trains are discussed in more detail below in the detaileddescription section, which includes discussion of the differencesbetween the present invention and the prior art direct drive compressionassemblies for air separation plants.

SUMMARY OF THE INVENTION

The present invention may be characterized as a method for compressionof an incoming feed air stream comprising the steps of: (a) compressingat least a portion of the incoming feed stream in a lower pressuresingle stage or multi-stage compression unit of a common air compressiontrain, at least one compression stage in the lower pressure single stageor multi-stage compression unit driven directly by a first variablespeed driver assembly; and (b) further compressing the compressed streamin one or more higher pressure single stage or multi-stage compressionunits of the common air compression train, wherein the at least one ofthe higher pressure single stage or multi-stage compression units aredriven by a second variable speed driver assembly; (c) purifying thefurther compressed feed stream to remove impurities either after step(a); after step (b) or between compression stages of step (b). Thevolumetric flow of the incoming feed stream is controlled by adjustingthe speed of a primary driver assembly selected from the first variablespeed driver assembly or the second variable speed driver assembly inresponse to changes in the operating conditions of the cryogenic airseparation plant and wherein a ratio of the speed of the variable speeddriver assemblies prior to such adjustment is different than the ratioof the speed of the variable speed driver assemblies after adjustment.In addition, the efficiency of the air separation plant at the selectedvolumetric flow can be optimized or changed by adjusting the speed ofthe first variable speed driver assembly or the second variable speeddriver assembly or both in response to changes in the operatingconditions or constraints of the cryogenic air separation plant.

From a compression train control standpoint, the volumetric flow of theincoming feed air stream is preferably controlled by adjusting the speedof the first variable speed driver assembly in response to changes inthe operating conditions of the cryogenic air separation plant such thata discharge pressure from the common air compression train is a variabledischarge pressure that changes by adjusting the speed of the firstvariable speed driver assembly and/or the second variable speed driverassembly in response to changes in the operating conditions of thecryogenic air separation plant. Operating conditions of the plant mayinclude such conditions as turndown conditions or even ambient airconditions.

Other aspects of the compression train control is to adjust the speed ofthe second variable speed driver assembly based, in part on the speed ofthe first variable speed driver assembly. For example, the speed of thefirst variable speed driver assembly may be set in response to ameasured flow rate of air in the common air compression train and thespeed of the second variable speed driver assembly may be set inresponse to a measured pressure of at least one of the portions ofpurified, compressed air streams in the split functional air compressiontrain in conjunction with the speed of the first variable speed driverassembly. Alternatively, the speed of the second variable speed driverassembly may be set in response to a discharge pressure in the commonair compression train and the speed of the first variable speed driverassembly.

Another control option is to control the speed of the first variablespeed driver assembly in response to the measured flow rate of air inthe common air compression train and one or more process limits,compressor limits, or driver assembly limits. The speed of the secondvariable speed driver assembly would also be set or adjusted in responseto similar process limits, compressor limits, or driver assembly limitsin conjunction with the speed of the first variable speed driverassembly.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims specifically pointing outthe subject matter that Applicants regard as the inventions, it isbelieved that the subject matter of the inventions will be betterunderstood when taken in connection with the accompanying drawings inwhich:

FIG. 1 is a schematic flow diagram of a cryogenic air separation plantincorporating one of the preferred methods for the compression of anincoming feed air stream in a cryogenic air separation plant inaccordance with the present invention;

FIG. 2 is a schematic flow diagram of a cryogenic air separation plantincorporating another of the preferred methods for the compression of anincoming feed air stream in a cryogenic air separation plant inaccordance with the present invention;

FIG. 3 is a schematic flow diagram of a cryogenic air separation plantincorporating yet another of the preferred methods for the compressionof an incoming feed air stream in a cryogenic air separation plant inaccordance with the present invention;

FIG. 4 is a schematic flow diagram of a cryogenic air separation plantincorporating an alternative arrangement for the compression of anincoming feed air stream in a cryogenic air separation plant inaccordance with the present invention;

FIG. 5 is a schematic flow diagram of a cryogenic air separation plantincorporating another alternative arrangement for the compression of anincoming feed air stream in a cryogenic air separation plant inaccordance with the present invention;

FIG. 6 is a schematic flow diagram of a cryogenic air separation plantincorporating yet another alternative arrangement for the compression ofan incoming feed air stream in a cryogenic air separation plant inaccordance with the present invention;

FIG. 7 is a schematic flow diagram of a cryogenic air separation plantincorporating a third alternative arrangement for the compression of anincoming feed air stream in a cryogenic air separation plant inaccordance with the present invention;

FIG. 8 is a schematic flow diagram of a cryogenic air separation plantincorporating another variant of the third alternative arrangement forthe compression of an incoming feed air stream in a cryogenic airseparation plant in accordance with the present invention;

FIG. 9 is a schematic flow diagram of a cryogenic air separation plantincorporating yet another variant of the third alternative arrangementfor the compression of an incoming feed air stream in a cryogenic airseparation plant in accordance with the present invention;

FIG. 10 is a schematic flow diagram of an air compression trains in acryogenic air separation plant illustrating aspects and features for thecontrol of the air compression trains in accordance with the presentinvention;

FIG. 11 is a schematic flow diagram of an air compression train in acryogenic air separation plant illustrating further aspects and featuresfor the control of such air compression trains in accordance with thepresent invention;

FIG. 12 is a schematic flow diagram of an air compression train in acryogenic air separation plant illustrating yet further aspects andfeatures for the control of such air compression trains in accordancewith the present invention; and

FIG. 13 is a schematic, fragmentary view of the sacrificial rigid shaftcoupling arrangement between a motor shaft and an impeller.

DETAILED DESCRIPTION

As used herein, the phrase Common Air Compression (CAC) train means aplurality of compression stages, intercoolers, aftercoolers andpre-purification units that are configured to compress, cool, andpre-purify substantially all of an incoming feed air stream to aprescribed flow, pressure, and temperature condition. The common aircompression train would typically include compressors in the MACcompression arrangement (or pre-MAC arrangement) and optionally one ormore initial compression stages of the BAC compression arrangement,wherein each of the compressors within the common air compression trainare configured for compressing substantially all of the incoming feedair stream.

As used herein, the phrase Split Functional Air Compression (SFAC) trainmeans a plurality of compression stages, intercoolers, aftercoolers,turbo-expanders that compress, cool, and/or expand selected portions ofthe compressed, pre-purified air stream from the prescribed condition totwo or more split streams having flow, pressure, and temperatureconditions suitable for: (i) boiling liquid products from thedistillation column system, (ii) producing cold turbine and/or warmturbine refrigeration for the distillation column system, and (iii)rectification in the distillation column system. The split functionalair compression train would typically include one or more latercompression stages of the BAC compression arrangement; compressorsassociated with any cold turbine refrigeration circuits such as an uppercolumn turbine (UCT) air circuit and lower column turbine (LCT) aircircuit; compressors associated with warm recycle refrigeration circuitssuch as a warm recycle turbine (WRT) air circuit, or other downstreamcompression stages configured for compressing less than substantiallyall of the compressed air stream from the common air compression train.

The term or phrase ‘integrally geared compressor’ (IGC) means one ormore compression stages coupled to a single speed driver assembly, and agearbox adapted for driving the one or more of the compression stagesvia a bull gear and associated pinion shafts such that all pinion shaftsoperate at constant speed ratios. For electric motor driven IGCs, thesingle speed is defined by the motor speed whereas in steam turbinedriven IGCs, the single speed is preferably characterized as a verysmall speed range that is dependent on the steam turbinecharacteristics. In contrast, the term or phrase ‘direct drivecompressor assembly’ (DDCA) means one or more compression stages drivenby a variable speed driver assembly and that does not include a gear boxor transmission.

Prior to providing a detailed discussion of the multiple embodiments ofthe present inventions, the subject matter of the present inventions maybe better understood through comparison to conventional IGC basedcompression trains as well as comparison to some of the closest priorart direct driven compression assemblies discussed in the paragraphsthat follow.

Most main air compression systems for cryogenic air separation plantsrequire some type or form of air flow control. Conventionally, this airflow control involves adjustment of the inlet guide vanes (IGV) on oneor more of the compression stages of an integrally geared compressor(IGC), and preferably the lowest pressure compression stage of acentrifugal air compressor of the MAC compression train. Alternate airflow control techniques or methods for air separation plants usingconventional IGC include suction/discharge throttling, recirculation ofthe air, or venting of the air flow. IGVs are typically considered anefficient method of air flow control of a centrifugal air compressorbecause at a given speed of the IGC, the IGV reduces the air flow to thecompression stage while the discharge pressure is maintained atacceptable levels. The overall isothermal efficiency of the IGCcompressor with IGV based control is higher when compared to otherconventional methods for compressor air flow control such assuction/discharge throttling or recirculation/venting. However, IGVbased control alone on a typical centrifugal compressor are not asefficient in turn down conditions compared to an air compression systemhaving compression stages driven by two or more variable speed motorssuch as the present systems and methods described herein.

Fixed or single speed operation, used in most IGC based compressionsystems with or without IGV's, can be used to control air flow (i.e.flow˜speed) but the discharge pressure decreases more rapidly withreductions in IGC driver speed (i.e. pressure˜speed²) giving a quadraticrelationship between pressure and flow (i.e. pressure˜flow²). Ingeneral, this type of quadratic relationship between flow and pressurein a conventional IGC based system is not an ideal match for an airseparation process. This quadratic relationship between pressure andflow however, is matched in a more efficient and beneficial manner usingan air compression system having at least two variable speed motors,preferably operating at different motor speeds and motor speed ratios.Thus, air flow control using two variable speed motors in a cryogenicair separation plant (e.g. as shown in FIGS. 1-3) have severaladvantages over the conventional IGC based compression systems.

The advantages include the turn-down capabilities and turn-downefficiency of an air compression system using two variable speed motorsin a cryogenic air separation plant compared to conventional IGC basedcompression systems using IGV's for air flow control. Table 1 comparesthe turndown capability and isothermal compression efficiency of atypical IGC machine using IGVs versus a direct drive compressionassembly (DDCA) based air compression system having two variable speedmotors without IGVs.

TABLE 1 Turndown Discharge IGC Compression DDCA Compression SystemConditions System with IGV with two variable speed motors TurndownIsothermal Isothermal Discharge Efficiency Efficiency Motor #1 Motor #2Air Flow Pressure Penalty IGV Penalty Speed Speed (% relative (%relative (% relative position (% relative (% relative (% relative toDesign to Design to Design (% of full to Design to Design to DesignCase) Case) Case) range) Case) Case) Case) 100 100 0.0 3 0.0 100.0 100.0(Design (Design Case) Case) 95 97 0.6 35 0.1 98.0 100.0 90 94 1.6 54 0.295.2 100.9 85 92 2.9 69 0.4 92.0 102.2 80 89 4.3 82 0.7 88.6 103.8 75 875.5 98 1.1 85.3 105.1 70 85 Operation not possible 1.5 82.3 105.9 65 83due to Surge, IGV or 2.0 80.1 105.8 60 82 other compressor limits 2.580.0 104.5 55 80 (without venting excess 3.1 80.0 101.6 50 79 compressedair) 3.8 80.0 99.0

As seen in Table 1, a cryogenic air separation plant using the typicalIGC based compression system with IGVs on the lowest pressurecompression stage for air flow control typically cannot turn down bymuch more than about 25%. Plant turn down operating conditions requiringair flows between about 50% to 70% of the design air flow for theconventional IGC based compression systems will often encounter externalsystem constraints or equipment constraints (e.g. surge conditions,surge margin, IGV limits, compressor limits, etc.) unless remedialactions are taken such as venting of excess compressed air. In addition,a relatively large isothermal efficiency penalty of up to about 5.5% ormore is realized when turn-down of a typical IGC based compressionsystem using IGVs is required.

In comparison, a cryogenic air separation plant using a DDCA basedcompression system having two variable speed motors has a turn downcapability of up to about 50% of the design air flow before encounteringexternal system constraints or equipment constraints with a much smallerisothermal efficiency penalty. Such turn down is achieved by adjustingthe speeds of the two variable speed motors. As described in more detailbelow, the speed of the second variable speed motor preferably isadjusted based, in part, on the speed of the first variable speed motor.Furthermore, since two manipulated variables (i.e. motor 1 speed andmotor 2 speed) are available to control, it is possible to adjust thetwo motor speeds to maintain higher average wheel efficiency for avariety of air flows compared to the conventional IGC based centrifugalair compressor arrangement having only IGV control. In addition to theturndown capability and turndown efficiency benefits described above,this DDCA) based compression system having two variable speedmotors—having two manipulated variables allows for an optimalre-stacking of the compression stages over their usable ranges therebyachieving a net improvement in the delivered compression efficiency.

Adjustment of the DDCA discharge pressure or some other system pressureallows the plant operator to: (i) expand the possible operationalenvelope of the air separation plant in terms of achievable productslate; (ii) avoid compressor limitations and constraints such as surgeconditions or pressure limits in the downstream functional aircompression train or downstream common air compression train; and/or(iii) adjust operational characteristics of downstream turbines, etc.Addition of other manipulated variables such as a third variable speedmotor and/or IGVs to the above-described DDCA can also serve to increaseair separation plant efficiency, turndown capability, turndownefficiency, and/or expansion of the air separation plant operationalenvelope.

In patent publication WO 2011/017783, a high-pressure multistagecentrifugal compressor arrangement is disclosed. This Atlas-Copcocompression arrangement includes four separate compressor elements orstages driven by two high speed electric motors. However, in one of thedisclosed arrangements in WO 2011/017783, there are two initialcompression stages arranged in parallel and driven directly by twoseparate high speed electric motors, wherein the two initial compressionstages are configured to receive and compress ambient pressure air toproduce a first and a second compressed air stream that are combined anddirected in a serial arrangement with two subsequent compression stages.Each of the two subsequent compression stages is also driven directly bythe same high speed electric motors driving the parallel initialcompression stages. Specifically, the first high speed electric motordrives compression stage 1 (i.e. compression of ambient air) andcompression stage 4 whereas the second high speed electric motor drivescompression stage 2 (compression of ambient air) and compression stage3. An alternative arrangement disclosed in WO 2011/017783 suggests allfour of said compressor elements could be placed in series connectionforming four consecutive stages with the first high speed electric motordriving a first low-pressure compressor element and a third compressorelement of the third pressure stage, while the second high speedelectric motor is driving the second compressor element as well as thefourth compressor element of the last stage.

The advantage of both arrangements disclosed in WO 2011/017783 is toprovide a uniform load distribution over both high speed electricmotors. However, a disadvantage of these Atlas-Copco compressionarrangements is realized in that by adjusting the speed of the firsthigh speed electric motor to control the air flowrate through thecompression system, it also directly impacts the final dischargepressure from the total compression arrangement. In other words, the airflowrate and discharge pressure from this compression arrangement areinherently and inseparably linked and controlled together when adjustingthe speed of the first high speed electric motor. Changing the speed ofthe first high speed electric motor also directly affects the dischargepressure from downstream compression stage 3 or compression stage 4 ofthe compression train. Also, the disclosed Atlas-Copco arrangement wherecompression stages 1 and 2 are in parallel requires identical control ofthe first and second high speed electric motors to achieve the desiredbalance loads.

Another similar high-pressure multi-stage centrifugal compressorarrangement is disclosed in another Atlas-Copco owned patent document,namely U.S. Pat. No. 7,044,716. This compressor arrangement containsthree compressor elements which are arranged in series as compressorstages, and at least two high speed electric motors to drive these threecompressor elements. Specifically, the low pressure stage is driven by afirst high speed electric motor where the high pressure stages (i.e.compression stage 2 and stage 3) are driven by a second high speedelectric motor. As taught in this patent, the Atlas-Copco direct drivecompression arrangement replaces the single high pressure stage of aconventional IGC arrangement with two high-pressure stages which aredriven by one and the same high-speed motor. By splitting thehigh-pressure stage in two stages, the pressure ratio per stage isreduced, so that the required rotational speed of the high-speed motoris also reduced. This design further allows the pressure ratios to beselected such that the specific speeds of the high-pressure compressionstages do not deviate much from the optimal specific speed.

Another closely related prior art reference is U.S. Patent ApplicationPublication No. 2007-0189905 which discloses a multi-stage compressionsystem that includes a plurality of centrifugal compression stages witheach stage having an impeller coupled to and driven by a variable speedelectric motor. The multi-stage compression system also includes acontrol system that is connected to each of the variable speed motorsand is operable to vary the speed of each motor such that the speed ofeach motor is varied simultaneously and that the ratio of the speed ofthe variable speed motors remains constant.

While the prior art references described above each disclose embodimentsof a direct drive compression arrangement, none of the disclosed priorart arrangements are particularly suited for use in the compressiontrain of large air separation plants. Thus, none of the above-identifieddirect drive compression arrangements disclose all of the elements andfeatures of the air separation compression train disclosed and claimedherein.

Specifically, none of the prior art references identified above discloseintermediate compression stages disposed between the compression stagesdriven directly by the variable speed motors. Similarly, none of theprior art references identified above disclose or teach subsequentcompression stages disposed downstream of direct driven compressionstages to further compress the incoming feed air stream in a common aircompression train or portions of the incoming feed air stream in a splitfunctional air compression train. Furthermore, none of the prior artreferences identified above disclose compression stages directly drivenby the second variable speed motor are configured to further compress areduced volumetric flow of the feed air stream in the split functionalair compression train.

In addition, none of the prior art references identified above discloseembodiments where the control of second variable speed motor is based,in part on speed of first electric motor or wherein a ratio of the speedof the variable speed motors is not maintained constant, as disclosed inthe embodiments of the present invention.

Compression Train Arrangements

Turning to FIG. 1, there is shown a schematic flow diagram of acryogenic air separation plant 10. An incoming feed air stream isfiltered in an air suction filter house (not shown) which is typically afree standing structure with a plurality of hooded intakes, each havingtwo or more stages of filtration made up of a plurality of filter panelsper stage. The filtered incoming feed air stream 12 is then compressedin a lower pressure compressor unit 17 of the compression arrangement,which forms the initial compression stage of the common air compressiontrain 20 to produce a first compressed air stream 14. The lower pressurecompressor unit 17 is driven directly by a first variable speed driverassembly, shown as a first high speed and variable speed electric motor15. The first compressed air stream 14 is cooled in intercooler 13 andthen directed to a second compressor unit 19 of the compressionarrangement, which forms the second compression stage of the common aircompression train 20 and which is also driven directly by the firstvariable speed electric motor 15 to produce a second compressed airstream 16. Neither, either or both of the first lower pressurecompressor unit 17 and the second compressor unit 19 may have inletguide vanes 21 to assist in the control of the air flow through thecommon air compression train 20.

The second compressed air stream 16 is again cooled in intercooler 23and directed to a third compressor unit 27 of the compressionarrangement which forms the third compression stage of the common aircompression train 20 to produces a third compressed air stream 22 andwhich is driven directly by a second variable speed drive assembly,shown as a second variable speed electric motor 25. After furthercooling in another intercooler 23 to remove the heat of compression, thethird compressed air stream 22 is still further compressed in a fourthcompressor unit 29 of the compression arrangement, which forms thefourth compression stage of the common air compression train 20 and afourth compressed air stream 24 and which is also driven directly by thesecond high speed, variable speed electric motor 25. Again, neither,either or both of the third and fourth compressor units 27, 29 may haveinlet guide vanes 31 to assist in the control of the air flow throughcommon air compression train 20.

Following the main air compression stages, the compressed feed airstream 24 is typically cooled and chilled using a direct contactaftercooler 43 or alternatively an indirect heat exchanger. Such directcontact aftercooler 43 is preferably designed with a low pressure dropand with high capacity packing to minimize capital cost and energylosses associated with the direct contact aftercooler 43. Theaftercooler 43 is also designed to extract water droplets from thecompressed feed air stream through the use of a demister (not shown) toensure that any water mist or water droplets are not carried through tothe pre-purification unit 35, which could adversely impact the airseparation plant by deactivating the drying sieves in thepre-purification units.

The pre-purification unit 35 is an adsorptive based system configured toremove impurities such as water vapor, hydrocarbons, and carbon dioxidefrom the feed air stream. Although the pre-purification unit 35 is showndisposed downstream of the fourth compressor unit 29 of the common aircompression train 20, it is contemplated that one can place thepre-purification unit 35 further upstream in the common air compressiontrain 20. The pre-purification unit 35 generally consists of at leasttwo vessels containing layers of different molecular sieves that aredesigned to remove the impurities from the compressed feed air stream24. While one vessel is active in removing such contaminants andimpurities, the other vessel and adsorbent beds disposed therein arebeing regenerated.

The regeneration process is a cyclic, multi-step process involving stepsoften referred to as blowdown, purge, and repressurization. Blowdown ofthe vessel involves releasing or changing the vessel pressure from thehigh feed pressure maintained during the active adsorptive process to apressure close to ambient pressure levels. The adsorbent bed is thenpurged or regenerated at the lower pressure using a waste gas producedby the distillation column system. After regeneration, thepurged/regenerated bed is repressurized from the near ambient pressureto the higher feed pressure by diverting a portion of the compressedfeed air stream 32 from the main air compression train to the vesseluntil it is repressurized.

In addition to periodically diverting a portion of the compressed feedair stream 32 for purposes of pre-purification unit repressurization,there may be times where diversion of clean dry air from the common aircompression train 20 downstream of the pre-purification unit is requiredfor other portions of the plant or there may be times where venting aportion 36 of the compressed air stream 24 upstream of thepre-purification unit is required for the safe operation of the airseparation plant 10 or to de-ice the air suction filter house. To thatend, a repressurization circuit 33 and valve 34 as well as otherdiversion circuits or vent circuits 37 and associated valves 38 areshown in the figures.

Further compression of most or substantially all of the compressed andpurified feed air stream 28 in one or more further compression stagesdisposed downstream of the pre-purification unit 35 may also beemployed. Such downstream compressor units 39 or compression stages maybe configured to be part of an integrally geared compressor 50 or may beyet another direct drive machine. As these compression stages 39 aredisposed downstream of the pre-purification unit 35, they are generallyconsidered part of the boosted air compression train, which is separatefrom the main air compression train, but as described herein, may remainpart of the common air compression train 20. Use of intercoolers and/oraftercoolers 41 disposed between or after the compression stages servesto keep the further compressed and purified feed air stream atappropriate temperatures through the common air compression train 20.

The compressed, purified and cooled feed air stream 30 exiting thecommon air compression train 20 is then directed to a split functionalair compression train 60 having one or more compression stages 65,67.However, rather than compressing the entire compressed, purified andcooled feed air stream 30, the split functional air compression train 60divides the stream into two or more portions 62, 64. As seen in FIG. 1,one portion of the compressed and purified feed air stream is referredto as boiler air stream 62 that is optionally compressed in compressorunit 65 and the resulting further compressed stream 66 cooled in cooler41 and fed to the primary heat exchanger 70 and used to boil liquidproducts produced by the air separation plant 10, such as liquid oxygen,to meet the gaseous product requirements. The cooled, compressed boilingair stream 66 is further cooled in the primary heat exchanger 70 viaindirect heat exchange with the liquid oxygen stream to form a liquidair stream 72 at temperatures suitable for rectification in thedistillation column system 80 of the cryogenic air separation plant 10.As seen in the Figures, the liquid air stream 72 is often split into twoor more liquid air streams, 74, 75 with a first portion of the liquidair stream 74 directed to the higher pressure column 82 and anotherportion of the liquid air 75 being directed to the lower pressure column84. Both liquid air streams 74, 75 are typically expanded using anexpansion valves 76, 77 prior to introduction into the respectivecolumns.

Another portion of the compressed and purified feed air stream is oftenreferred to as a turbine air stream 64 that is optionally compressed incompressor unit 67 with the resulting further compressed stream 68 beingpartially cooled in the primary heat exchanger 70. The compressed andpartially cooled turbine air stream 69 is then directed to a turbine aircircuit 90 where it is turbo-expanded in a turbo-expander 71 to providerefrigeration to the cryogenic air separation plant 10, with theresulting exhaust stream 89 being directed to distillation column system80 of the cryogenic air separation plant 10. The turbine air circuit 90illustrated in FIG. 1 is shown as a lower column turbine (LCT) aircircuit where the expanded exhaust stream 89 is fed to the higherpressure column 82 of the distillation column system 80. Alternatively,the turbine air circuit may be an upper column turbine (UCT) air circuitwhere the turbine exhaust stream is directed to the lower pressurecolumn. Still further, the turbine air circuit may be a warm recycleturbine (WRT) air circuit where the turbine exhaust stream is recycledwithin a refrigeration loop coupled to the primary heat exchanger, orother variations of such known turbine air circuits such as a partiallower column turbine (PLCT) air circuit or a warm lower column turbine(WLCT) air circuit.

Each of the compression stages disposed downstream of thepre-purification unit 35 may be configured to be part of an integrallygeared compressor (IGC) 50 or may be coupled to and driven by the shaftwork of the turbo-expanders. In such cases, the compression stagespreferably include a bypass circuit 55 and by-pass valve 57 the flowthrough which is controlled to prevent or mitigate unwanted conditionsin the compression stage such as a surge condition, margin limit,stonewall condition or excessive vibration condition, etc.

As indicated above, one or more of the portions of the compressed andpurified feed air stream 66, 68 in the split functional air compressiontrain 60 are passed through the primary heat exchanger 70 andsubsequently introduced or fed to the distillation column system 80 ofthe cryogenic air separation plant 10 where the air streams areseparated to produce liquid products 92, 93; gaseous products, 94, 95,96, 97; and waste streams, 98. As well known in the art, thedistillation column system 80 is preferably a thermally integratedtwo-column or three column arrangement in which nitrogen is separatedfrom the oxygen to produce oxygen and nitrogen-rich product streams. Athird column or an argon column 88 can also be provided that receives anargon-rich stream from the lower pressure column 84 and separates theargon from the oxygen to produce an argon containing product 96. Oxygenthat is separated from the feed air stream can be taken as a liquidproduct 92 that can be produced in the lower pressure column as anoxygen-rich liquid column bottoms 91. Liquid product 93 can additionallybe taken from part of the nitrogen-rich liquid 99 used in refluxing oneor more of the columns. As known in the art, the oxygen liquid productcan be pumped via pump 85 and then taken, in part, as a pressurizedliquid oxygen product 92, and also heated, in part, in the primary heatexchanger 70 against the boiler air stream 66 to produce a gaseousoxygen product 94 or as a supercritical fluid depending on the degree towhich the oxygen is pressurized by the pumping. The liquid nitrogen cansimilarly be pumped and taken as either as pressurized liquid product, ahigh pressure vapor or a supercritical fluid.

In many regards, the embodiment illustrated in FIG. 2 is similar to theembodiment of FIG. 1 with one key difference, namely the lower pressurecompression stage or compressor unit 17 is driven by a dedicated firstvariable speed electric motor 15. As with the above embodiments, thelower pressure compressor unit 17 may also include inlet guide vanes 21to assist in the control of the incoming feed air stream flow throughthe common air compression train 20. The subsequent two compressionstages in the common air compression train 20 arranged in series withthe initial or lower pressure compression stage are driven by the secondvariable speed electric motor 25. Still further compression stages orcompression units 39 of the common air compression train 20 as well asthe compression stages or compression units 65, 67 in the splitfunctional compression train 60 are preferably part of one or moreintegrally geared compressors (IGC) 50 or may be driven by the shaftwork of the turbo-expanders. In this embodiment, the downstreamcompressor unit 39 of the common air compression train 20, as well asthe additional intercooler 43 are situated upstream of thepre-purification unit 35.

Likewise, the embodiment illustrated in FIG. 3 is also similar to theembodiment of FIG. 1 with another key difference, namely there are twolower pressure compression stages or compressor units 17A, 17B arrangedin parallel both driven by the first variable speed electric motor 15.The subsequent two compression stages or compressor units 27, 29 in thecommon air compression train 20 are driven by the second variable speedelectric motor 25 and arranged in series with the two lower pressurecompression stages. Still further compression stages or compressor units39A, 39B of the common air compression train 20 as well as any optionalcompression stages in the split functional compression train (not shown)are preferably part of one or more integrally geared compressors (IGC)50 or may be driven by the shaft work of the turbo-expanders. As shownin FIG. 3, the two lower pressure compression stages 17A, 17B preferablyhave a common air feed 11 through which the two centrifugal compressorstages 17A,17B are fed with ambient pressure filtered air 12 and acommon outlet 18 from which the compressed air is discharged as a firstcompressed air stream 14. The first centrifugal compressor stage 17A ispreferably mounted on one end of a motor shaft of the variable speedelectric motor 15 while the second centrifugal compressor stage 17B ismounted on the other end of the motor shaft. Neither, either or both ofthe first and the second centrifugal compressors have inlet guide vanes21. Alternatively, this arrangement may be configured such that each ofthe two lower pressure compression stages each receive and compressdifferent volumetric flows of ambient pressure air. Such alternativearrangement may provide certain operational and cost advantages duringturndown of the air separation plant 10.

Turning now to FIG. 4, there is shown a schematic flow diagram of acryogenic air separation plant 110 employing another variant of thecommon air compression train 120 having two or more variable speeddriver assemblies 115, 125. As with the earlier described embodiments,the incoming feed air stream 112 is filtered and then compressed in thelower pressure compressor unit or stage 117 of the compressionarrangement, which forms the initial compression stage of the common aircompression train 120 to produce a first compressed air stream 114. Thelower pressure compressor unit or stage 117 is driven directly by afirst variable speed driver assembly, shown as a first high speed andvariable speed electric motor 115. The first compressed air stream 117is cooled in intercooler 113 and directed to a second compressor unit orstage 119 of the compression arrangement, which forms the secondcompression stage of the common air compression train 120 which is alsodriven directly by the first variable speed electric motor 115 toproduce a second compressed air stream 116. Neither, either or both ofthe first compressor unit/stage 117 and the second compressor unit/stage119 may have inlet guide vanes 121 to assist in the control of thecommon air compression train 120.

In the embodiments shown in FIGS. 4-6, the second compressed air stream116 is again cooled in intercooler 123 and directed to one or moreintermediate compression stages in the form of an additional compressorunits/stages 124. Unlike the lower pressure compressor units 117, 119,these additional compressor units/stages 124 need not be driven by avariable speed driver assembly, but rather, more preferably are part ofan integrally geared compressor (IGC) 150. However, the latercompression stages of the common air compression train 120 include oneor more higher pressure compression stages 127, 129 are driven by asecond high speed, variable speed electric motor 125.

Similar to the earlier described embodiments, the embodiments shown inFIGS. 4-6 also include a pre-purification unit 135, a plurality ofintercoolers 123, aftercoolers 143 in the common air compression train120 as well as any required bypass circuits 155, bypass valves 157,diversion or vent streams 136 and circuits 137, and repressurizationstreams 132 and circuits 133 and associated valves 134, 138 thatfunction in a manner described with reference to FIGS. 1-3. Theembodiments further include a primary heat exchanger 170 and a twocolumn or three column distillation column system 180 (including anoptional argon column 188 configured to produce an argon containingproduct 196) where the purified air streams are separated to produceliquid products 192, 193; gaseous products, 194, 195, 196, 197; andwaste streams, 198. Oxygen that is separated from the incoming air feedcan be taken as a liquid product 192 that can be produced in the lowerpressure column as an oxygen-rich liquid column bottoms 191. Liquidproduct 193 can additionally be taken from part of the nitrogen-richliquid 199 used in refluxing one or more of the columns. The oxygenliquid product can be pumped via pump 185 and then in part taken as apressurized liquid product 192, and also heated in the primary heatexchanger 170 against the boiler air stream 166 to produce a gaseousoxygen product 194.

The compressed, purified and cooled feed air stream 130 exiting thecommon air compression train 120 in FIGS. 4-6 is then directed to asplit functional air compression train 160 having and one or morecompression stages or compressor units 165, 167. However, rather thancompressing the entire compressed, purified and cooled feed air stream130, the split functional air compression train 160 divides the stream130 into two or more portions 162, 164. As seen in the drawings, oneportion of the compressed and purified feed air stream is referred to asboiler air stream 166 that is compressed in compressor unit 165, cooledin cooler 141 and fed to the primary heat exchanger 170 where it is usedto boil liquid oxygen products to meet the gaseous oxygen productrequirements of the plant 110. The boiling air stream 166 portion of thefeed air stream is sufficiently cooled in the primary heat exchanger 170via indirect heat exchange with the pumped liquid oxygen stream 191 toform a liquid air stream 172 at temperatures suitable for rectificationin the distillation column system 180 of the cryogenic air separationplant 110. The liquid air stream 172 is often split into two or moreliquid air streams with a portion of the liquid air stream 174 directedto the higher pressure column 182 and another portion of the liquid airstream 175 being directed to the lower pressure column 184. Both liquidair streams 174, 175 are typically expanded using an expansion valves176, 177 prior to introduction into the respective columns.

Another portion of the compressed and purified feed air stream is oftenreferred to as a turbine air stream 168 that is optionally compressed incompressor unit 167 and partially cooled in the primary heat exchanger170. The partially cooled and compressed turbine air stream 169 isdirected to a turbine air circuit 190 where it is expanded inturbo-expander 171 to provide refrigeration to the cryogenic airseparation plant 110, with the resulting exhaust stream 189 beingdirected to distillation column system 180 of the cryogenic airseparation plant 110. The turbine air circuit 190 illustrated in FIG. 4is shown as a lower column turbine (LCT) air circuit where the expandedexhaust stream 189 is fed to the higher pressure column 182 of thedistillation column system 180. However, as described above, the turbineair circuit may be an upper column turbine (UCT) air circuit where theturbine exhaust stream is directed to the lower pressure column, a warmrecycle turbine (WRT) air circuit where the turbine exhaust stream isrecycled within a refrigeration loop coupled to the primary heatexchanger, or variations of such known turbine air circuits such as apartial lower column turbine (PLCT) air circuit or a warm lower columnturbine (WLCT) air circuit.

In many regards, the embodiment illustrated in FIG. 5 is similar to theembodiment of FIG. 4 but where the lower pressure compression stage orcompressor unit 117 is driven by a dedicated first variable speedelectric motor 115. As with the above embodiments, the lower pressurecompressor unit 117 may also include inlet guide vanes 121 to assist inthe control of the incoming feed air stream flow through the common aircompression train 120. The subsequent two intermediate pressurecompression stages 125A, 125B in the common air compression train 120arranged in series with the initial or lower pressure compression stage117 or stages and are preferably part of one or more integrally gearedcompressors (IGC) 150 whereas one or two of the later higher pressurecompression stages 127, 129 of the common air compression train 120 aredriven by the second variable speed electric motor 125 in either asingle ended configuration (i.e. one higher pressure compression stage)or double ended configuration (i.e. two higher pressure compressionstages). Any downstream compression stages 165, 167 in the splitfunctional compression train 160 are also preferably part of one or moreintegrally geared compressors (IGC) 150 or may be driven by the shaftwork of the above-described turbo-expanders.

Likewise, the embodiment illustrated in FIG. 6 is also similar to theembodiment of FIG. 4 with two lower pressure compression stages 117A,117B arranged in parallel that are both driven by the first variablespeed electric motor 115. The subsequent two intermediate pressurecompression stages 125A, 125B in the common air compression train 120are preferably part of one or more integrally geared compressors (IGC)150 whereas the one or two later higher pressure compression stages 127,129 of the common air compression train 120 are located downstream ofthe pre-purifier unit 135 and driven by the second variable speedelectric motor 125 in either a single ended configuration (i.e. onehigher pressure compression stage) or double ended configuration (i.e.two higher pressure compression stages). In this embodiment, the twolower pressure compression stages comprise two centrifugal compressorsor compression units/stages 117A, 117B preferably have a common air feed111 through which the two centrifugal compressors are fed with ambientpressure air 112 and a common outlet 118 from which the compressed air114 is discharged. The first centrifugal compressor unit/stage 117A ispreferably mounted on one end of the motor shaft of the first variablespeed electric motor 115 while the second centrifugal compressorunit/stage 117B is mounted on the other end of the motor shaft. Neither,either or both of the first and the second centrifugal compressors mayhave inlet guide vanes 121.

Turning now to FIG. 7, there is shown a schematic flow diagram of acryogenic air separation plant 210 employing a third variant of the airseparation compression train having two or more variable speed driverassemblies 215, 225. As with the earlier described embodiments, theincoming feed air stream 212 is compressed in the lower pressurecompressor unit 217 of the compression arrangement, which forms theinitial compression stage of the common air compression train 220 toproduce a first compressed air stream 214. The lower pressure compressorunit 217 is driven directly by the first variable speed driver assembly,shown as a first high speed and variable speed electric motor 215. Thecompressed air stream 214 is cooled in intercooler 213 and directed to asecond compressor unit 219 of the compression arrangement, which formsthe second compression stage of the common air compression train 220which is also driven directly by the first variable speed electric motor215 to produce a second compressed air stream 216. Neither, either orboth of the first compressor unit 217 and the second compressor unit 219may have inlet guide vanes 221 to assist in the control of the commonair compression train 220.

The remaining compression stages of the common air compression train 220including one or more intermediate pressure compression stages 224A,224B and one or more higher pressure compression stages need not bedriven by a variable speed driver assembly, but rather, more preferablyare part of an integrally geared compressor (IGC) 250. Similar to theearlier described embodiments, the embodiments shown in FIGS. 7-9 alsoinclude a pre-purification unit 235, a plurality of intercoolers 223,aftercoolers 243 in the common air compression train 220 as well as anyrequired bypass circuits 255, bypass valves 257, diversion or ventstreams 236 and circuits 237, and repressurization streams 232 andcircuits 233 and associated valves 234, 238 that function in a mannerdescribed above with reference to FIGS. 1-3. The embodiments furtherinclude a primary heat exchanger 270 and a two column or three columndistillation column system 280 (including an optional argon column 288configured to produce an argon containing product 296) where thepurified air streams are separated to produce liquid products 292, 293;gaseous products, 294, 295, 296; and waste streams, 297, 298. Oxygenthat is separated from the incoming air feed can be taken as a liquidproduct 292 that can be produced in lower pressure column 284 as anoxygen-rich liquid column bottoms 291. Liquid product 293 canadditionally be taken from part of the nitrogen-rich liquid 299 used inrefluxing one or more of the columns. The oxygen liquid product can bepumped via pump 285 and then in part taken as a pressurized liquidproduct 292, and also heated in the primary heat exchanger 270 againstthe boiler air stream 266 to produce a gaseous oxygen product 294.

The compressed, purified and cooled feed air stream exiting the commonair compression train 220 in FIGS. 7-9 is then directed to a splitfunctional air compression train 260. Specifically, the split functionalair compression train 260 divides the compressed and purified air streaminto two or more portions. As seen in FIG. 7, one portion of thecompressed and purified feed air stream is referred to as boiler airstream 266 that is still further compressed in a one or two boiler aircompressor units 265A, 265B that includes one or more higher pressurecompression stages driven by the second variable speed driver assemblyor, more particularly, the second high speed, variable speed electricmotor 225. The second variable speed drive assembly 225 may beconfigured as a single ended arrangement (i.e. one higher pressureboiler air compression stage 265A) or double ended arrangement (i.e. twohigher pressure boiler air compression stages 265A, 265B).

The further compressed boiler air stream portion 266 is fed to theprimary heat exchanger 270 and used to boil liquid oxygen to meet thegaseous oxygen product requirements of the air separation plant 210. Theboiling air stream portion 266 of the feed air stream is sufficientlycooled in the primary heat exchanger 270 via indirect heat exchange withthe liquid oxygen stream to form a liquid air stream 272 at temperaturessuitable for rectification in the distillation column system 280 of thecryogenic air separation plant 210. The liquid air stream 272 is oftensplit into two or more liquid air streams with a portion of the liquidair stream 274 directed to the higher pressure column 282 and anotherportion of the liquid air stream 275 being directed to the lowerpressure column 284. Both liquid air streams 274, 275 are typicallyexpanded using an expansion valves 176, 277 prior to introduction intothe respective columns.

Another portion of the compressed and purified feed air stream is oftenreferred to as a turbine air stream 268 that is optionally compressed incompressor unit 267 and partially cooled in the primary heat exchanger270. If further compressed, the turbine air compression stages 267 arepreferably part of an integrally geared compressor (IGC) 250 or may becoupled to and driven by the shaft work of the turbo-expanders.

The partially cooled turbine air stream 269 is directed to a turbine aircircuit 290 where it is expanded using turbo-expander 271 to providerefrigeration to the cryogenic air separation plant 210, with theresulting exhaust stream 295 being directed to distillation columnsystem 280 of the cryogenic air separation plant 210. The turbine aircircuits 290 illustrated in FIGS. 7-9 are shown as lower column turbine(LCT) air circuits where the expanded exhaust stream 295 is fed to thehigher pressure column 282 of the distillation column system 280.Alternatively, the turbine air circuits may be upper column turbine(UCT) air circuits where the turbine exhaust stream is directed to thelower pressure column, warm recycle turbine (WRT) air circuits where theturbine exhaust stream is recycled within a refrigeration loop coupledto the primary heat exchanger, or variations of such known turbine aircircuits such as partial lower column turbine (PLCT) air circuits orwarm lower column turbine (WLCT) air circuits.

The embodiment illustrated in FIG. 8 is similar to the embodiment ofFIG. 7 but where the lower pressure compression stage or compressor unit217 is driven by the dedicated first variable speed electric motor 215.As described above, the lower pressure compressor unit 217 may alsoinclude inlet guide vanes to assist in the control of the incoming feedair stream flow through the common air compression train 220. Thesubsequent intermediate pressure compression stages 224A, 224B andhigher pressure compression stages 239 in the common air compressiontrain 220 are arranged in series with the initial or lower pressurecompression stage 217 and are preferably part of one or more integrallygeared compressors (IGC) 250. Alternatively, one or more of theintermediate pressure compression stages and higher pressure compressionstages may be coupled to and driven by the shaft work of theturbo-expanders.

In the embodiment of FIG. 8, the boiler air stream 262 portion of thecompressed and purified feed air stream is further compressed in aboiler air compressor unit 265 driven by the second high speed, variablespeed electric motor 225. In addition one or more turbine aircompressors 267 may be coupled to and driven by the second variablespeed drive assembly 225. The second variable speed drive assembly 225is configured either as a single ended configuration (i.e. for a boilerair compression stage 265 only) or a double ended configuration (i.e.for the boiler air compression stage 265 and a turbine air compressionstage 267).

Likewise, the embodiment illustrated in FIG. 9 is also similar to theembodiment of FIG. 7 with two lower pressure compression stages 217A,217B arranged in parallel that are both driven by the first variablespeed electric motor 215. The subsequent intermediate pressurecompression stages 224A, 224B and higher pressure compression stages (ifany) in the common air compression train 220 are arranged in series withthe initial or lower pressure compression stages 217A, 217B and arepreferably part of one or more integrally geared compressors (IGC) 250.Alternatively, one or more of the intermediate pressure compressionstages and higher pressure compression stages may be coupled to anddriven by the shaft work of the turbo-expanders. In this embodiment ofFIG. 9, the two lower pressure compression stages 217A, 217B comprisetwo centrifugal compressors or compression units preferably have acommon air feed 211 through which the two centrifugal compressors arefed with ambient pressure air 212 and a common outlet 218 from which thecompressed air 214 is discharged. The first centrifugal compressor 217Ais preferably mounted on one end of the motor shaft of the firstvariable speed electric motor while the second centrifugal compressor217B is mounted on the other end of the motor shaft. Either or both ofthe first and the second centrifugal compressors may have inlet guidevanes 221.

Further, all or part of the boiler air stream portion 266 of thecompressed and purified feed air stream in the split functional aircompression train 260 is further compressed in one or two boiler aircompressors driven by the second high speed, variable speed electricmotor 225. The boiler air compressors 265A, 265B may be coupled to anddriven by the second variable speed electric motor 225 in a single endedconfiguration (i.e. for one boiler air compression stage) or in a doubleended configuration (i.e. for two boiler air compression stages). Inlieu of driving the boiler air compressors with the second variablespeed electric motor, an alternative arrangement similar to that shownin FIGS. 7-9 is contemplated using two turbine air compressors arrangedin parallel or in series are coupled to and driven by the secondvariable speed electric motor.

Compression Train Control

From a compression train control standpoint, a useful metric is theoverall cryogenic air separation plant efficiency. Independentlyadjusting the speeds of the first and second variable speed motorassemblies in view of external constraints and plant operatingconditions enables changes to the overall cryogenic air separation plantefficiency, and more preferably increases in the overall cryogenic airseparation plant efficiency.

The cryogenic air separation plant efficiency is a specific measure ofhow efficiently the desired slate of products can be generated by theair separation unit given all of the internal and external constraintsand operating conditions of the plant. This cryogenic air separationplant efficiency and the desired product slate are impacted by andsubject to one or more of the following considerations: 1) airseparation unit operating cost factors, 2) value of produced products,3) market demand for produced products, 4) pre-existing contractualobligations and penalties, 5) equipment and process constraints, etc.The possible slate of products can include one or more of the followingproducts, including gaseous oxygen, gaseous nitrogen, liquid nitrogen,liquid oxygen, argon, clean dry air, steam, etc. The product slate isheavily dependent on the air separation unit system and process designas well as external considerations such as customer demands, contractualobligations for product delivery, market demand and prices, productavailability from other air separation units on the same or other plantsites.

The cryogenic air separation plant efficiency measure is also highlydependent on operating cost implications of the air compression system,including the common air compression system as well as the splitfunctional air compression system which are used to deliver a desiredair flow at a suitable pressures, or within a range of suitablepressures. Total air compression system operating costs depend on thetotal usage of air compression system power and other operating costconsiderations such as cooling water cost, pre-purification regenerationenergy costs, maintenance costs, etc. The most significant of theseoperating cost factors are air compression system power, which can be inthe form of electricity, steam, natural gas, etc., and the net powerprice, which can be a fixed power price, a contractually set powerprice, or a real-time variable power price, and may or may not includepower distribution cost charges and fees. The total usage of aircompression system power is further dependent on the net efficiency ofthe individual compressors, turbines and other equipment in the commonair compression system and split air compression system from which thenet efficiency of the total air compression system for the airseparation unit can be ascertained. Monitoring air flows, product streamflows, and other system stream flows as well as system/equipmentpressures, pressure ratios, and temperatures allows the plant operatorto better understand the operating constraints and resistances andcontrol the speeds of the variable speed motor assemblies to change thecryogenic air separation plant efficiency, and preferably increase theoverall cryogenic air separation plant efficiency for a given air flowand product slate.

FIGS. 10-12 depict embodiments of the air compression train within anair separation plant showing some control features associated with thevarious components of the air compression trains. As seen therein, thespeed of the first variable speed motor 315 is a control parameter thatis set and/or adjusted based on a first command signal 301 correspondingto the first motor assembly limits (JIC) 302, a command second signal303 via the flow indicated control (FIC) 304 corresponding to themeasured flow rate of air in the common air compression train asmeasured using a flow measurement device 371, and a third command signal305 corresponding to any manual indicated controls (HIC) 306 oroverrides from the plant operator. A selector 307, such as a lowselector (<), compares the three command signals and selects theappropriate input 308 to the drive assembly to set and/or adjust thespeed for the first variable speed electric motor 315 to compress theincoming feed air stream 312. Similarly, the speed of the secondvariable speed motor 325 is a control parameter that is set and/oradjusted based on a command signal 341 corresponding to the second motorassembly limits via the equipment indicated controller (JIC) 342, anymanual indicated controller (HIC) 344 or overrides from the plantoperator and a third command signal 345 produced by a controller 350that is based on the signal 310 corresponding to the speed of the firstvariable speed electric motor 315, a signal 346A corresponding to themeasured discharge pressure in the air compression train via thepressure indicated controller (PIC) 347A, 347B, and a signal 348corresponding to the measured flow rate of air in the common aircompression train via the flow indicated control (FIC) 349. A selector340, such as a low selector (<), compares the three command signals 341,343, 345, and selects the appropriate input 352 to drive assembly to setand/or adjust the speed 354 for second variable speed electric motor325.

In the illustrated embodiments, the measured discharge pressure in theair compression train is a measured pressure in the turbine air circuitof the split function air compression train via the pressure indicatedcontroller (PIC) 347A or 347B situated upstream of the primary heatexchanger 380 and turbo-expander 390. Alternative pressure indicatedcontrols may be in the boiler air circuit of the split function aircompression train or at various locations in the common air compressiontrain. For example, use of pressure indicated controllers forintermediate discharge pressures from each pair of commonly drivencompression stages or intermediate discharge pressures from eachindividual stage may be used to limit the speeds of either or bothvariable speed motors. Such pressure indicated controls or other manualindicated controls may also be used to control other aspects of the aircompression train in conjunction with the above-described controlmethods such as control of turbine nozzles 392 associated with one ormore turbo-expanders or control of inlet guide vanes 394 associated withany compressor units in the common air compression train or splitfunction air compression train.

For example, pressure indicated controls 316 corresponding to thepressure of the compressed air stream 314 between compression stagesdriven by the first variable speed motor 315 may be used as an input tocontrol the speed of the first variable speed motor 315 (See FIG. 11) orused to control the inlet guide vanes 394 of the associated compressorunits 317, 319 (See FIG. 12). Likewise, pressure indicated controls 326corresponding to the pressure of the compressed air stream 322 betweencompression stages driven by the second variable speed motor 325 may beused as inputs 318, 328 to control the speed of the first variable speedmotor 315 and the second variable speed motor 325, respectively (SeeFIG. 11) or used to control the inlet guide vanes 394 of the associatedcompressor units 327, 329. (See FIG. 12). Also, manual indicated control395 and/or pressure indicated controller 347B can be used to control theturbine nozzle 392 position via signals 396 and 346B respectively, asthe desired position is preferably correlated with the dischargepressures in the common air compression train and/or the splitfunctional air compression train (see FIG. 11). Alternative embodimentscontemplate the use of other inter-stage pressures and/or the pressureratios across each compression stage may be used to set the speeds ofeither or both of the first and second variable speed motors to achievea net improvement in delivered air compression efficiency.

Surge indicated controllers (UIC) 360, 362, are also associated witheach of the first and second variable speed driver assemblies, and morespecifically with one or more of the compressor units 317, 319, 327, and329 driven by the variable speed driver assemblies. The surge indicatedcontrollers (UIC) 360, 362 preferably use some form of flow measurementand pressure to estimate surge or the on-set of a surge condition. Toprevent the surge condition, the surge indicated controllers (UIC) 360,362 are directed to a selector 361 that opens the vent 338 to dischargea portion of the compressed air 336 as to avoid the surge condition inone or more of the compressor units driven by the variable speed driverassemblies. Similar surge indicated controllers (UIC) 370, 372, 374 mayalso be used in operative association with other compression stages orcompressor units 365, 367, 369 both in the common air compression trainas well as in the split functional air compression train. To prevent thesurge condition in those downstream compressor units 365, 367, 369, thesurge indicated controllers (UIC) 370, 372, 374 open bypass valves 375,377, 379 associated with respective compressor unit so as to avoid thesurge condition.

As illustrated, the preferred compression train control involvesadjusting the speed of the second variable speed driver assembly based,in part, on the speed of the first variable speed driver assembly. Inaddition to or in lieu of basing the speed control of the variable speedmotors on the motor assembly limits, another control option is tocontrol the speed of the first variable speed driver assembly inresponse to the measured flow rate of air in the common air compressiontrain and one or more plant process limits, compressor limits, or otherdriver assembly limits. The speed of the second variable speed driverassembly would also be set or adjusted in response to similar plantprocess limits, compressor limits, or other driver assembly limits inconjunction with the speed of the first variable speed driver assembly.

Other external constraints or equipment constraints may also beintegrated into the air compression train control. For example, if thefirst variable speed motor encounters a constraint, such as speedconstraint, then the speed of the second variable speed motor can beadjusted to maintain the desired air flowrate through the common aircompression train in addition to or in lieu of its' default controlvariable. Other constraints that would require the second variable speedmotor to control flowrate include surge conditions, surge margin,stonewall conditions, pressure, torque, power, etc.

Put another way, during normal operations the second variable speedelectric motor is controlled using the speed of the first variable speedelectric motor together with a secondary variable to achieve the desiredpressure and temperature conditions of the compressed air streams. Thesecondary variable may include discharge pressure, as shown in FIGS.10-12 or other selected variable such as a speed setpoint, powersetpoint, motor speed ratios, discharge pressure ratios, power ratios,etc. Normal operations would typically mean that the first variablespeed electric motor is adjusted to fully control the primary controlvariable, which is preferably the incoming feed air stream flowrate.

Non-normal operations, on the other hand, means that the primary motorspeed cannot be used to achieve full control of the primary controlvariable because some system or external constraint is encountered. Suchconstraints may include one or more system process limits such as apressure, pressure ratio, temperature, etc.; one or more compressionstage limits such as a compressor wheel surge condition, margin limit,stonewall condition, vibration condition, etc.; or one or more driverassembly limits such as speed limitation, torque limitation, powerlimitation, bearing conditions, motor operating temperatures, andvibration conditions. Non-normal operations can also result from otherair separation plant or process conditions. During non-normal operationsthe speed of the second variable speed electric motor is controlledusing the speed of the first variable speed electric motor to achievethe desired incoming feed air stream flowrate in view of the system orexternal constraint.

In conventional DDCA based compression systems or IGC based compressionsystems, individual compressor loadings are often designed or selectingso as to balance the loadings between the parallel arranged compressorssuch that the compressor loadings are not optimized toward powerreduction. As a result, the unit compression power for such parallelarranged compressors is typically higher than the minimum unitcompression power.

To address this disadvantage, the preferred control system may alsoemploy the use of model predictive controls to provide real-timeadjustment of the compressor loading of parallel arranged compressorsand optimum flow distribution between two parallel arranged compressorsin the common air compression train (see FIGS. 3, 6, and 9). Suchparallel compressor optimization via model predictive control ispreferably targeted to reduce the net power consumption rather thanbalancing the compressor loading using an equal flow, approach to surgeor equivalent scheme. A typical parallel compressor optimizationequation is shown generically as:

${\min\limits_{F_{1},F_{2}}{Power}} = {{k_{F_{1},1}F_{1}} + {k_{F_{1},2}F_{1}^{2}} + {k_{F_{2},1}F_{2}} + {k_{F_{2},2}F_{2}^{2}}}$

where the total flow (F_(total)) is the sum of the flow to a firstparallel compressor (F₁) and a second parallel compressor (F₂), k arevalues ascertained from characterizations and modeling of the specificcompressors, and the optimization routines are subject to specificcompressor constraints or limitations including: F₁>F_(1,surge);F₂>F_(2,surge); F₁<F_(1,max); and F₂>F_(2,max).

Sacrificial Rigid Shaft Coupling

In all of the aforementioned embodiments, the high speed electric motorassemblies each having a motor body, a motor housing, and a motor shaftwith one or more impellers directly and rigidly coupled to the motorshaft using a sacrificial rigid shaft coupling. As shown in FIG. 13, thesacrificial rigid shaft coupling 500 is provided with a coupling body400 which includes opposed first and second ends 402 and 404. Thecoupling is connected at the first of the ends 402 to the impeller 432and at the second of the ends 404 to the motor shaft 416. The couplingbody 400 has a deformable section 406 highlighted in the dashed circlethat will deform under a desired unbalanced loading exerted against thecoupling body upon failure of the impeller 432 allowing it topermanently deform and do so without the deformable section 406exceeding the ultimate strength of a material forming the coupling body400 and to limit the unbalanced load force and moment to preventpermanently deforming the motor shaft 416 and which can result in afailure of the journal bearings. In this regard, such a material couldbe a high ductility metal, with yield strength sufficiently large tohandle normal design loads, yet sufficiently low to limit unbalancedload forces and moments from permanently deforming the motor shaft,meanwhile the combination of elastic and ultimate strength allow theimpeller to touch the shroud without cracks occurring in the coupling.Such a material could be 15-5PH (H1150) stainless steel.

As illustrated, section deformable 406 has a sufficiently large annularshaped area, as viewed in an outward radial direction thereof that witha given material is sufficient to transmit the torque from the motorshaft 416 to the impeller 432 during normal intended operation. It isalso a short section as viewed in an axial direction parallel to themotor shaft 416 so as to be sufficiently stiff as not to allowundesirable motor shaft vibrations during such normal operation.However, in case of a failure of the impeller 432, the section 406 isdesigned to undergo a stress that will exceed the elastic limit of thematerial making up the coupling and thereby deform without exceeding theultimate strength or ultimate limit of such material. As a result ofsuch deformation the first of the ends 402 of the coupling 500 willbegin to rotate in a clockwise direction with the end result of theimpeller 432 striking the shroud of compressor. Put another way, thecoupling sacrifices itself by yielding in section 406 for the sake ofthe motor. After a failure of the coupling, the motor will not have apermanently deformed shaft 416 and potentially have reusable bearings.The motor will still be able to be used and the arrangement can berenewed by refurbishment of the compressor.

Deformable section 406 is produced by providing the coupling body 400with an axial bore 408 that has a wider portion 410 inwardly extendingfrom the second of the ends 404 toward the first of the ends 402 and anarrow portion 412 extending from the wider portion 410 toward thesecond of the ends 402. This results in the coupling body having areduced wall thickness “t” at a location along axial bore 408 that willact as a weak point at which the coupling body 400 will deform. Thus,deformable section 406 forms a juncture between the wider and narrowerportions 410 and 412 of the axial bore 408. Typically, failure of theimpeller will be due to the loss or partial loss of an impeller blade432 a. The deformable section is then designed to fail or in other wordsdeform as a result of a certain imbalance and under a loading producedat an operational motor speed. At the same time sufficientcross-sectional area must be provided to allow torque transmission andvibration during normal operation. As can be appreciated, other designscould be used in producing deformable section or a sacrificial rigidshaft coupling. For example, if the axial bore 408 were of constantdiameter, an outer circumferential groove-like portion within thecoupling body 400 could produce such a deformable section.

As seen in FIG. 13, the connection between impeller 432 and the coupling500 is preferably a clutch type toothed coupling 414 provided by aninterlocking arrangement of teeth. The teeth are provided both at thefirst of the ends 402 of the coupling body 400 and also on a hub 417 ofthe impeller 432. This clutch type toothed coupling has many variationsand names but, is typically referred to as a “HIRTH” type of coupling.In order to maintain contact and provide torque transmission, apreloaded stud 418 can be connected to coupling 500 by a threaded typeconnection 419 within the narrower section 412 of the axial bore 408 ofthe coupling body 400. A nut 420 threaded onto the stud 418 holds thehub 417 of the impeller 432 against the first of the ends 402 of thecoupling body 400 and therefore, the clutch type toothed coupling 414 inengagement. As can be appreciated by those skilled in the art, numerousother means could be provided for connecting the impeller 432 to thecoupling 500, for instance a friction, keyed, polygon, or interferencefit.

The connection between motor shaft 416 and the second of the ends 404 ofthe coupling 500 is provided by an annular flange-like section 422 ofthe coupling body 400 surrounding the wider portion 410 of the axialbore 408. A set of preloaded screws 424 pass through the flange-likesection 422 and are threadably engaged within bores (not shown) providedin the end of the motor shaft 416. Preferably the coupling body 400 hasan annular projection 428 that seats within a cylindrical, inwardlyextending recess 430 situated at the end of the motor shaft 416 tocenter the coupling body 400 with respect to the motor shaft 416. Thisprovides better centering of impeller4 32 with shaft 416 and helps inthe assembly thereof.

Preferably, rotating labyrinth seal elements 432 and 434 are part of thecoupling 500 and as illustrated, are provided on exterior portions ofthe annular flange-like section 422 and the first of the ends 402 of thecoupling body 400. These elements engage complimentary labyrinth sealelements situated on the shaft seal 443 within a housing of the electricmotor adjacent the impeller 432. By placing both the necessary processgas shaft seal and the rotor air gap cooling stream shaft seal on thecoupling, impeller overhang is minimized and the chances of creating arigid rotor and preferable rotor dynamics is allowed. The seals, whiletypically rotating labyrinths, could be a brush or carbon ring seal. Asecondary benefit of minimizing impeller overhang is that should damageto the seals occur, which can occasionally happen, only the couplingneeds replacing. This is in contrast to seals typically located on therotor which would need renovation or replacement. Shaft seal 443 formsthe stationary sealing surfaces between rotating labyrinth seals 432 and434 which control the motor cooling gas leakage flow and compressorprocess gas leakage flow, respectively. The motor cooling gas leakageflow and compressor process gas leakage flow combine to form a totalleakage flow which generally exits from a passage 440 in volute.

While the present invention has been described with reference to apreferred embodiment or embodiments and operating methods associatedtherewith, it is understood that numerous additions, changes andomissions to the disclosed systems and methods can be made withoutdeparting from the spirit and scope of the present inventions as setforth in the appended claims.

What is claimed is:
 1. A method for controlling the compression of anincoming feed stream to a cryogenic air separation plant, the methodcomprising the steps of: (a) compressing at least a portion of theincoming feed stream in a lower pressure single stage or multi-stagecompression unit of the common air compression train, at least onecompression stage in the lower pressure single stage or multi-stagecompression unit driven directly by a first variable speed driverassembly; (b) further compressing the compressed stream in one or morehigher pressure single stage or multi-stage compression units of thecommon air compression train, wherein the at least one of the higherpressure single stage or multi-stage compression units are driven by asecond variable speed driver assembly; and (c) purifying the furthercompressed feed stream to remove impurities either after step (a); afterstep (b) or between compression stages of step (b); wherein a volumetricflow of the feed stream is controlled by adjusting the speed of aprimary driver assembly selected from the first variable speed driverassembly or the second variable speed driver assembly in response tochanges in the operating conditions of the cryogenic air separationplant and wherein a ratio of the speed of the variable speed driverassemblies prior to such adjustment is different than the ratio of thespeed of the variable speed driver assemblies after adjustment; andwhereby the efficiency of the cryogenic air separation plant changes byadjusting the speed of the first variable speed driver assembly or thesecond variable speed driver assembly or both in response to changes inthe operating conditions of the cryogenic air separation plant.
 2. Themethod of claim 1 wherein the efficiency of the cryogenic air separationplant increases by adjusting the speed of the variable speed driverassemblies.
 3. The method of claim 1 further comprising the step ofdirecting portions of the compressed and purified feed air stream to asplit functional air compression train having one or more boostercompression stages.
 4. The method of claim 3 wherein the speed of thefirst variable speed driver assembly is set in response to a measuredflow rate of air in the common air compression train and wherein thespeed of the second variable speed driver assembly is set in response toa measured pressure of at least one of the portions of purified,compressed air streams in the split functional air compression train andthe speed of the first variable speed driver assembly.
 5. The method ofclaim 1 wherein the speed of the first variable speed driver assembly isset in response to a measured flow rate of air in the common aircompression train and wherein the speed of the second variable speeddriver assembly is set in response to a discharge pressure in the commonair compression train and the speed of the first variable speed driverassembly.
 6. The method of claim 1 wherein the speed of the firstvariable speed driver assembly is set in response to the measured flowrate of air in the common air compression train and one or more processlimits and wherein the speed of the second variable speed driverassembly is set in response to the one or more one or more processlimits and the speed of the first variable speed driver assembly.
 7. Themethod of claim 1 wherein the speed of the first variable speed driverassembly is set in response to the measured flow rate of air in thecommon air compression train and one or more compression stage limitsand wherein the speed of the second variable speed driver assembly isset in response to the one or more compression stage limits and thespeed of the first variable speed driver assembly.
 8. The method ofclaim 1 wherein the speed of the first variable speed driver assembly isset in response to the measured flow rate of air in the common aircompression train and one or more driver assembly limits and wherein thespeed of the second variable speed driver assembly is set in response tothe one or more one or more driver assembly limits and the speed of thefirst variable speed driver assembly.
 9. The method of claim 1 whereinthe speed of the first variable speed driver assembly is set in responseto the measured flow rate of air in the common air compression train andone or more limits selected from the group of process limits,compression stage limits, and driver assembly limits, and wherein thespeed of the second variable speed driver assembly is set in response tothe speed of the first variable speed driver assembly and one or morelimits selected from the group of process limits, compression stagelimits, and driver assembly limits.
 10. The method of claim 1 whereinthe speed of the first variable speed driver assembly or the speed ofthe second variable speed driver assembly or both are further adjustedperiodically in response to the diversion or venting of a portion of thecompressed air from the common air compression train.
 11. The method ofclaim 1 wherein the speed of the first variable speed driver assembly orthe speed of the second variable speed driver assembly or both arefurther adjusted in response to changes in ambient air conditions. 12.The method of claim 1 wherein the speed of the second variable speeddriver assembly is adjusted in response to changes in the operatingconditions of the cryogenic air separation plant and the speed of thefirst variable speed driver assembly and wherein a ratio of the speed ofthe variable speed driver assemblies prior to such adjustment isdifferent than the ratio of the speed of the variable speed driverassemblies after adjustment.
 13. A method for controlling thecompression of a feed air stream to a cryogenic air separation planthaving a common air compression train and a split functional aircompression train, the method comprising the steps of: (a) compressingat least a portion of an incoming feed stream in a lower pressure singlestage or multi-stage compressor of the common air compression train, atleast one compression stage in the lower pressure single stage ormulti-stage compressor driven directly by a first variable speed driverassembly; and (b) further compressing the compressed stream in one ormore higher pressure single stage or multi-stage compressors of thecommon air compression train wherein the at least one of the higherpressure single stage or multi-stage compression units are driven by asecond variable speed driver assembly; wherein a volumetric flow of thefeed stream is controlled by adjusting the speed of a primary driverassembly selected from the first variable speed driver assembly or thesecond variable speed driver assembly in response to changes in theoperating conditions of the cryogenic air separation plant; wherein theefficiency of the air separation plant at the volumetric flow isoptimized by managing one or more operating pressures within the plantincluding one or more discharge pressures in the common air compressiontrain or in the split functional air compression train, and the one ormore operating pressures change by adjusting the speed of the firstvariable speed driver assembly or the second variable speed driverassembly or both in response to changes in the operating conditions ofthe cryogenic air separation plant and the speed of the primary driverassembly; and wherein during turndown conditions a ratio of the speed ofthe first variable speed driver assembly to the speed of the secondvariable speed driver assembly is between about 0.75 to 0.99.
 14. Themethod of claim 13, further comprising the steps of: further compressingthe compressed stream after step (a) in one or more intermediatepressure compressors of the common air compression train wherein the oneor more of the intermediate pressure compressors are driven by a thirdvariable speed driver assembly; and wherein the speed of the thirdvariable speed driver assembly is adjusted, in part, in response to thespeed of the first and second variable speed driver assemblies.
 15. Themethod of claim 13, further comprising the steps of: further compressingthe compressed stream after step (b) in one or more higher pressurecompressors of the split functional air compression train wherein theone or more of the higher pressure compressors of the split functionalair compression train are driven by a third variable speed driverassembly; and wherein the speed of the third variable speed driverassembly is adjusted, in part, in response to the speed of the first andsecond variable speed driver assemblies.
 16. The method of claim 13,further comprising the steps of: directing all or a portion of the feedair stream into inlet guide vanes before step (a), the inlet guide vanesbeing operatively associated with the lower pressure compressor; andwherein the position of the inlet guide vanes are adjusted, in part, inresponse to the speed of the first and second variable speed driverassemblies.